1. Field of the Invention
The present invention relates to a power conversion device that converts AC power and DC power, and more particularly relates to a PWM controlled power conversion device wherein the harmonic components contained in the AC input current waveform are decreased and the efficiency of power conversion is improved.
2. Description of the Related Art
A power conversion device that converts AC power to DC power is the PWM controlled AC/DC power conversion device described in Laid-open Japanese Patent Publication No. H. 10-66343.
The essentials thereof are shown in FIG. 1, numeral 1 being an AC power source, numeral 3 being a self-excited voltage converter, numeral 4-1 and 4-2 being smoothing capacitors, numeral 5 being the load; and numeral 10 being a PWM controller. Self-excited voltage converter 3 is an NPC (neutral point clamped) circuit.
Typically in a PWM controlled AC/DC power conversion device, sine-wave PWM control is performed based on triangular wave comparison; however, by setting the modulation frequency (triangular wave carrier frequency) of PWM control higher than the AC power source frequency, the harmonic components contained in the input current waveform on the AC power source side can be reduced.
FIG. 2 shows an example of the waveform of such operation, and shows the sine-wave signal SIN, triangular wave carrier signals TR1, TR2, the switching signals Gu1, Gu2, Gx1, Gx2 that are applied to the U-phase switching devices Su1, Su2, Sx1, Sx2 of the self-excited voltage converter, and the AC-side U-phase voltage waveform Vsu of the converter. The voltage of the smoothing capacitor is here taken to be Vd.
In a power conversion device (AC/DC power conversion device) in which AC power is converted to DC power, if a large amount of harmonic components are contained in the input current waveform on the AC power source side, this is a factor causing distortion of the AC system voltage and may have an adverse effect on other electrical equipment connected to the same AC system. An AC/DC power conversion device in which there are few power source harmonics is therefore sought.
PWM control is performed in the AC/DC power conversion device shown in FIG. 1, but in general in AC/DC power conversion equipment of the PWM controlled type, the modulation frequency of PWM control must be raised in order to reduce the amount of harmonic components contained in the AC input current waveform.
It is therefore an unavoidable requirement to raise the switching frequency of the switching devices; however, there is an upper limit to the switching frequency, depending on the characteristic of the elements of the switching element. In particular, sufficient harmonic reduction effect was sometimes not obtained at the upper limiting frequency of the GTO thyristor elements that are commonly used in high-capacity power conversion devices. Furthermore, raising the switching frequency tended to increase switching losses generated in the switching element and was associated with a drop in the power conversion efficiency of the power conversion device.
Accordingly, an object of the invention of the present application is to provide a power conversion device capable of achieving both improved power conversion efficiency and a reduction in the amount of harmonics of the power source, by realizing a system in which the harmonic components contained in the AC input current waveform can be reduced without raising the switching frequency of the AC/DC power conversion device.
Accordingly, one object of the present invention is to provide a novel power conversion device comprising:
an active current control means (unit) that calculates a phase angle reference value for determining the ON/OFF phase based on the AC power source voltage phase from the deviation with respect to the active current reference value of the active current that is input to the power conversion device; and a fixed pulse pattern generating means (unit) that controls the self-excited voltage power conversion device by generating a switching signal of fixed pulse pattern whose fundamental frequency is synchronized with the AC power source frequency, based on the phase angle reference value calculated by this active current control means(unit).
According to the invention of the present application, even if, for the fixed pulse pattern, a pulse pattern of the switching signal supplied to the self-excited voltage type power conversion device is selected such as to produce a desired value of the harmonic components, it becomes possible to control the active current supplied to the power conversion device from the AC power source to a desired value, thereby making it possible to reduce the harmonic components contained in the AC input current waveform without raising the switching frequency of the switching devices.
Further, in order to achieve the above object, according to the invention of the present application, there are provided:
n transformers whose primaries are connected in series with the AC voltage source and that shift the phase of the secondary voltage by 60xc2x0/n in each case; and self-excited voltage converters whose AC sides are connected to the respective secondary sides of these transformers; and a pulse pattern generator that produces pulse patterns wherein the phase of the switching signal that is applied to these respective self-excited voltage converters is shifted in each case by 60xc2x0/n but is otherwise the same.
With the present application, the amount of harmonic components contained in the AC input current waveform can therefore be reduced, thanks to the series multi-connection effect at the AC input terminal of the self-excited voltage converters without raising the switching frequency of the switching devices.